The present invention is directed toward an atmosphere controlled electric melting furnace for use on a variety of materials and more particularly for the melting of non-metallics in the production of mineral wool insulation.
Since the 1850's, mineral wool for thermal and acoustic insulation has been produced from a wide variety of raw materials, including blast furnace slags from copper, lead and iron production. To make mineral wool, these materials are remelted in fuel-fired cupola furnaces which are primitive devices offering little quality control, substantial air pollution and, in recent years, high operating cost because of the steep rise in the cost of coke, their principal fuel.
Careful and detailed studies of the reactions in large cupolas such as an iron blast furnace, decades of effort to establish optimum levels for all its parameters, and enormous increases in size (several recently commissioned units exceed 10,000 tons of iron per day, or 14,000 pounds per minute) have resulted in predictable quality and reasonable economy.
By contrast, the small cupola furnaces (.apprxeq.5 tons per hour) in use all over the world to produce molten non-metallics to be fiberized into mineral wool are small and inefficient. No economies of scale have been achieved, because mineral wool is bulky, and cannot be transported over great distances without absorbing the margin in freight costs. Further, the "spinners" used by most operators to fiberize the molten stream of slag discharging from the cupola are generally limited to 5 tons per hour per set, in present practice, and mounted one set per furnace, or "line".
As a result, the typical cupola currently in service to melt non-metallics for mineral wool is a water-cooled steel shell 6 to 7 feet in diameter and 15 to 25 feet high. It is by nature thermally inefficient, air polluting and high in operating cost. The quantities of particulate matter, sulphur and sulphur oxides in the top discharge of fume from the cupola require prohibitively high capital and maintenance costs to control, considering that only 5 tons per hour are melted.
The cupola's most important deficiency is its lack of control of the quality of the product. Residence time, in a molten state, of each increment of charge is very small, of the order of seconds in some cases or minutes at most. Modification of tapping temperature can only satisfactorily be achieved by charge additions, such as sand, to lower the melting point. Increase in melt rate can only be achieved by increasing the blast, with a consequent change in residence time and tapping temperature.
The ability of the spinning system to convert most of the cupola discharge into high quality product is a function of the surface tension of the molten stream, which in turn is affected by temperature, chemistry and viscosity. The inability of the cupola to control these variables results in poor average performance. Sometimes, when optimum fiberizing conditions are approached, a cupola/spinner combination converts a much higher percentage of its molten feed into high quality product, indicating that even modest control of the key melting variables will give significant improvement in yield.
Surface tension is a critical parameter in the fiberization process. The breakdown of the slag film into fibers is illustrated in FIG. 1. The spinning wheel produces a plane sheet of liquid slag 10 which is hit at right angles by a high velocity stream of air. The slag film 10 is deflected and is subjected to aero-dynamic instabilities which develop into waves propagating with increased amplitude in more or less tangential orientation.
At the leading edge of the sheet, half or full wavelengths of the molten material are detached by the impact of the air blast 12 and contract into ligaments 14 under the influence of surface tension. What then happens to these ligaments, i.e. whether they are converted into useful fiber 16, or shot 18 to be rejected, depends largely upon the temperature-viscosity relationship.
Since raw materials, particularly iron blast furnace slags, are in abundance as (mostly) waste matter, and mineral wool of good quality has high value as insulation, a number of attempts have been made over the last 20 years to find a more satisfactory melting method. These attempts have generally been based upon the use of an electric furnace for resistance, arc or induction melting of the charge, with a view to providing molten material which is controlled in terms of flow rate, temperature and composition, at a competitive cost.
Each of these attempts has failed, not because electric melting of slags is in itself particularly difficult, but because its achievement in a controlled fashion with any conventional electric furnace has proved uneconomical.
The source energy used to melt a ton of blast furnace slag by means of a 5 ton per hour cupola may be shown to be about 7 million BTUs. Because of lack of control of the temperature, chemistry and rate of the cupola discharge, an average of 45 percent of this melted material is wasted as shot and tailings, so the source energy required for the melting of 1 ton of product is approximately 12.5 million BTUs.
By contrast, under ideal conditions, the total heat required to raise 1 ton of iron blast furnace slag to tapping temperature is approximately 450 KWH, or 1.5 million BTUs. However, since the efficiency of a modern thermal power station is 37 percent at best, and transmission losses to the melting site will probably account for another 10 percent, the total source energy requirement to raise 1 ton of slag to tapping temperature is, under ideal conditions, 4.5 million BTUs. And therefore, in conventional 5 ton per hour electric furnace of 70 percent overall thermal efficiency, source energy required is 6.4 million BTUs per ton melted. Assuming that the improvement in control of tapping temperature, chemistry and rate due to conventional electric melting provides an increase in useful mineral wool product from the present 55 percent to 65 percent, the net source energy requirement for this electric melter is 9.8 million BTUs per ton of product.
In summary, the source energy required per ton of mineral wool product is approximately 20 percent more for current cupola practice than it is for conventional electric melting.
Expressed in economic terms, at $170 per ton of coke, and an average power cost in the United States of $0.028 per KWH (in 1979), the savings in energy cost indicated for conventional electric melting over cupola melting are approximately $10 per ton melted, or $18 per ton of product.
Unfortunately, these savings in energy cost are offset by the high cost of refractories in the conventional electric furnace, because molten slag and the presence of available oxygen will erode all known refractory lining systems, even carbon and graphite. Carbonaceous materials oxidize, or burn away increasingly rapidly as their temperatures rise above 900 degrees F. For example, industrial graphite loses 6 percent of its weight by oxidation when maintained at 1,100 degrees F in air for only two and a half hours. The melting point of iron blast furnace slag, depending upon composition, is 2,500 to 2,800 degrees F.